A lithium-sulfur battery is a secondary battery where a sulfur-based compound having a sulfur-sulfur bond is used as a positive electrode active material and a carbon-based material in which intercalation and deintercalation of an alkali metal such as lithium or a metal ion such as a lithium ion occur is used as a negative electrode active material. Electric energy is stored and generated by using an oxidation-reduction reaction where an oxidation number of sulfur is reduced while the sulfur-sulfur bond is broken during discharging, which is a reduction reaction, and the sulfur-sulfur bond is formed again while the oxidation number of sulfur is increased during charging, which is an oxidation reaction.
In the lithium-sulfur battery, in the case where a lithium metal is used as the negative electrode active material, an energy density is 3830 mAh/g, and in the case where sulfur (S8) is used as the positive electrode active material, the energy density is 1675 mAh/g, and thus the lithium-sulfur battery is a promising battery in terms of energy density. Further, there is a merit in that a sulfur-based material used as the positive electrode active material is a low-priced and environmentally-friendly material.
However, there is a problem in that since electric conductivity of sulfur is 5×10−30 S/cm and thus sulfur is close to a nonconductor, it is difficult to move electrons generated by an electrochemical reaction. Accordingly, an electric conductive material such as a carbon providing a smooth electrochemical reaction site needs to be used. In this case, there are problems in that in the case where the conductive material and sulfur are simply mixed to be used, sulfur flows out to an electrolyte during the oxidation-reduction reaction to reduce a battery life-span, and in the case where an appropriate electrolytic solution is not selected, lithium polysulfide that is a reduction material of sulfur is eluted, and thus sulfur does not participate in the electrochemical reaction any more.
Therefore, in order to reduce an outflow of sulfur to the electrolyte and increase electronic conductivity of an electrode including the sulfur, a mixing quality of carbon and sulfur needs to be improved.